A printing method by an ink-jet printer recording apparatus is a method in which an ink is jetted from a nozzle to allow the ink to adhere to a printing target material. The printing method is a method of printing in a state where the nozzle is not in contact with the printing target material, and in the method, printing can be performed on surfaces having irregular shapes, such as curved surfaces and corrugated surfaces. Thus, the printing method is expected to be applied in a wide variety of industrial fields.
Inks used in ink-jet printer recording apparatuses are roughly divided into solvent system inks, such as aqueous inks and solvent inks and non-solvent system inks, such as actinic-ray-curable inks and solid inks. The non-solvent system inks are more eco-friendly than the solvent system inks which require permeation of a solvent into a printing target material and a solvent evaporation step. Also, the non-solvent system inks quickly become tack-free due to their quick drying properties and are excellent in printability on printing target materials that do not or hardly absorb the solvent. In particular, actinic-ray-curable inks among the non-solvent system inks can form a coating film having high strength, chemical resistance, and toughness in addition to the aforementioned characteristics.
In recent years, light sources used in curing of actinic-ray-curable inks for ink-jet recording have been gradually switching from conventional metal halide lamps, high-pressure mercury-vapor lamps, and the like to low energy light emitting diode lamps (hereinafter abbreviated to “LED lamps”).
Among the LED lamps, LED lamps that emit light of wavelengths in the ultraviolet region (hereinafter abbreviated to “UV-LED lamps”) generate ultraviolet rays having peak wavelengths in the range of 350 to 420 nm. Thus, when such a UV-LED lamp is used as a light source to cure an ink, the used ink has to contain a photopolymerization initiator whose absorption wavelength is 350 to 420 nm. However, pigments and the photopolymerization initiators contained in such inks generally have a nature of absorbing ultraviolet rays and the like having wavelengths of 350 to 420 nm in many cases. Accordingly, when conventional pigment inks are irradiated with light using a UV-LED light source, the ink sometimes cannot be sufficiently cured.
Known actinic-ray-curable inks for ink-jet recording that are excellent in curability even when a UV-LED lamp is used as a light source include: an ink set containing a yellow ink composition, a magenta ink composition, a cyan ink composition, and black ink composition, characterized in that the ink compositions each contains a radical polymerizable compound (component A), a radical polymerization initiator (component B), and a colorant (component C), the component B contained in each of the ink compositions containing an acylphosphine oxide compound (component B-1) and a thioxanthone compound (component B-2), the ink compositions each containing the component B-1 at 3% by weight or more and less than 17% by weight based on the total amount of the ink composition, the magenta ink composition and the cyan ink composition each containing the component B-2 at 0.1% by weight or more and less than 2% by weight based on the total amount of the ink composition, the yellow ink composition and the black ink composition each containing the component B-2 at 2% by weight or more and less than 6% by weight based on the total amount of the ink composition (see, for example, PTL 1); and an actinic-ray-curable ink composition for ink-jet recording containing an actinic ray polymerizable compound and a photopolymerization initiator, characterized by (1) containing a polymerizable compound having one actinic ray polymerizable group at 60 to 95% by mass based on the total amount of the actinic ray polymerizable compound and a polymerizable compound having two or more actinic ray polymerizable groups at 5 to 40% by mass based on the total amount of the actinic ray polymerizable compound, (2) containing N-vinyl-2-caprolaclam at 1 to 15% by mass and isobornyl acrylate at 1 to 25% by mass based on the total amount of the actinic ray polymerizable compound, as the polymerizable compound having one actinic ray polymerizable group, and (3) containing a polymerizable compound having a vinyl ether group as the polymerizable compound having two or more actinic ray polymerizable groups (see, for example, PTL 2).
However, when the ink or the ink set is used to perform printing into a thickness of approximately 20 μm to 30 μm and curing is tried using the UV-LED lamp, light may be difficult to penetrate to the interior (depths) of the thick ink coating film, resulting in failure of rapid curing of the coating film.
The actinic-ray-curable ink sets for ink-jet recording are generally often used in an aspect where different colored inks are subjected to laminated printing (multicolored printing). In such a case, a coating film formed of a single color ink may have a thickness of about 10 μm to 20 μm, and a coating film formed by the multicolored printing may have a large thickness of about 10 μm to 30 μm in total.
Laminated printing of different colored inks using the ink set to form a coating film having a large thickness has sometimes resulted in non-uniformity in curing degrees of the ink coating film due to difference in curability across the inks.
A large difference in curing rates across different colored inks sometimes leads to a difference in curing rates of dots across the inks, resulting in a significant difference in the dot sizes after curing. Printed matter having a significant difference in the dot sizes may be insufficient in practice in terms of clearness or the like of the printed matter as compared with printed matter having relatively uniform large dots formed therein.